According to recently rumors on NR, the upcoming Nikon full frame mirrorless camera will be shipping about one month after Photokina 2018 (in October). While the official announcement will be before Photokina, this is same as rumored before.
In a recently interview with Nikon executive officer Kimito Uemura by NHK World Japan (Video above), Nikon executive confirmed they will bring new Nikon mirrorless cameras before Spring of 2019.
Nikon has filed several new lens patents for both 24mm f/1.8 full frame mirrorless lens and 24mm f/1.8 APS-C mirrorless lens in Japan. As rumored before, Nikon will announce new mirrorless cameras in late 2017 to early 2018.